


A Private Lesson

by cloudafterdark



Category: Dr. STONE (Anime), Dr. STONE (Manga)
Genre: Adopted Sibling Relationship, Age Difference, Consensual Underage Sex, Dead Dove: Do Not Eat, Discord: The Kingdom of Shipping (Dr. STONE), Explicit Sexual Content, First Time Blow Jobs, M/M, Masturbation, Mutual Masturbation, Sibling Incest, TKS - School's In Session, TKSTsukasaandUSAShipsMonth, TKSevents, TheKingdomofShipping, Underage Sex
Language: English
Status: Completed
Published: 2020-08-01
Updated: 2020-08-01
Packaged: 2021-03-05 22:27:40
Rating: Explicit
Warnings: Underage
Chapters: 1
Words: 2,265
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/25642783
Author URL: https://archiveofourown.org/users/cloudafterdark/pseuds/cloudafterdark
Summary: Senku stuck the pinky of his other hand in his ear and glanced away. "I was... playing with Taiju and Tsukasa outside. And Tsukasa pinned me down and was moving on top of me, and I started feeling strange... Something weird happened."Xeno's heart skipped about four beats before it thudded back to life painfully in his chest.Yeah, he was fucked.~~~For the TKS USA Ships Month event theme: School's In Session
Relationships: Ishigami Senkuu/Dr. Xeno
Comments: 5
Kudos: 62
Collections: TKS Tsukasa Ship Month & USA Ship Month - August 2020





	A Private Lesson

**Author's Note:**

> Read the tags. Seriously.

Xeno knew from the moment he met Senku he was fucked.

He'd been in and out of foster homes a few times when Byakuya met him, and accepted him with open arms unlike any he'd seen before. He was something of a troublemaker—mostly due to his friend Stan's bad influence—but Byakuya didn't seem to care.

Or, well, he _cared,_ but he didn't get _angry_ about it. He was a proponent of that _unconditional love_ stuff.

So when he brought home this little baby, no more than a year old, Xeno knew he was on the way out the door. Everyone loved babies; no one wanted a teenager around when they could have fun with a little kid.

But surprisingly, Byakuya didn't show any indication of wanting to get rid of Xeno. He actually... _relied_ on Xeno. Asked him to babysit, and paid him for it, despite the fact that Senku was legally his little brother. And Senku was an easy kid to take care of.

It was kind of nice, to be honest.

But as Senku got older—and _smarter—_ he started asking questions. Most of them were about rockets; Senku wanted his big brother’s help building a rocket that would actually make it to space. Though there were plenty of other topics Senku would ask him about, too.

Xeno was 23, still living at home to save on expenses while he attended graduate school to study aerospace engineering, when his 11 year old baby brother knocked on his bedroom door. "Xeno," Senku asked. "Can I come in?"

Xeno didn't exactly _want_ him to come in, but he paused the video he was watching and tugged the waistband of his pants up. "Come in, Senku," he called out as he wiped his hand on his bedsheets—tucking the bottle of lube under the blanket in the process.

Senku opened the door and walked inside, tugging at the hem of his shirt with one hand. He looked almost _shy,_ which was unusual for the brazen little dweeb. "What's up?" Xeno asked.

Senku stuck the pinky of his other hand in his ear and glanced away. "I was... playing with Taiju and Tsukasa outside. And Tsukasa pinned me down and was moving on top of me, and I started feeling strange... Something weird happened. It felt like pee, but when I went to the bathroom, there was white stuff in my underwear."

Xeno's heart skipped about four beats before it thudded back to life painfully in his chest.

 _Fuck._ Yeah, he was fucked.

"Is something wrong with me?" Senku asked, his voice unusually meek. When he lifted his head, his eyes glimmered with unshed tears, pulling at Xeno's heartstrings—and then reminding him of the dreams he tried so hard to forget in the light of day. Senku was his _brother,_ and if he acted upon those thoughts, he was nothing more than a monster.

He shook his head. "No, not at all. This just means you're growing up, Sen. Come here, I'll explain everything.” The relief on Senku’s face was palpable; he looked up at Xeno with wide, thankful eyes and wobbling lips.

Yep, Xeno was a monster. He was going to hell for this. “But, hey... this is a pretty _grown-up_ conversation, so you'll need to close and lock the door, okay? And don’t tell Dad about this." 

Nodding quickly, Senku turned and pushed the door shut, then flipped the lock on the handle. Xeno patted the bed next to him for Senku to sit down. "Good. So, let's start at the beginning... You know about your penis, right? That it's where you pee out of."

"Yeah, _obviously,"_ Senku replied. "I'm not a baby, I know that much."

"That's right, you aren't a baby. And you aren't a little kid anymore, either. You're well on your way to becoming a man." He smiled and ruffled Senku's hair—very aware that he was using the hand still covered in traces of lube. "So, there's something else that can come out of your penis, too. The scientific term is semen, or ejaculate, but it's usually just referred to as cum. You've started puberty, and now your body is capable of reproduction."

"Oh. Is this like that birds and bees talk the old man tried to have with me a few months ago?" Senku pulled a face. "I'm not one millimeter interested in any girls. I don't think I'll ever want to have kids of my own."

Xeno couldn't hold back a chuckle at that declaration. "That's fair. I'm much the same. I'm not interested in women at all, just men."

"So what does that mean? Does your body make... cum if you aren't trying to have a baby? But Tsukasa isn't a girl, even if he is growing his hair out."

God, it was cute to hear Senku say _cum_ like that. Xeno had to hold back a shudder at the thought of filling his little brother up with his seed. "Your body makes cum all the time. It gets stored in your balls—speaking of which, I haven't seen recently, have your balls dropped yet?"

"Y-yeah! Of course they have!" Senku cried out in protest.

"Hmm, I don't know if I believe you..." 

"Th-then I'll show you!" Senku hopped off the bed and shucked his shorts off. His tight little briefs followed and, sure enough, two plump little balls hung under his dick. A sparse coating of hair covered them, pale blonde interspersed with green. 

Xeno had to swallow heavily before he could continue. "Well, well... Guess you were right."

Senku crossed his arms over his chest and smirked. "Told you."

"Yes, you did. So, as I said, your cum gets stored in your balls, which means you need to drain it regularly to keep yourself healthy."

"Drain it?" Senku tilted his head in curiosity, eager to glean new information he hadn't had before. "How do I do that?"

"There are a few ways," Xeno explained. "But the easiest is to use your hand. Or you can have a friend help you."

Senku's eyes widened. "A... friend?" At Xeno's nod, he continued, "How would I do it? Either by myself or to have someone else?"

"I'll teach you, if you want."

"Really? Please!"

"Sure thing. Come sit on my lap, it'll be easier that way."

Xeno helped get Senku settled on his lap, legs spread wide apart. He had to bite back a groan at the way his little brother's sweet ass cheeks cupped his erection; instead, he focused on looking down over Senku's shoulder at his softened little cock. "Something that will help with draining your balls is a lubricant. Saliva will do in a pinch, but body-safe lubricants are the best. You can use baby oil, lotion, or something like this..." Xeno drew the bottle of lube out from under his blanket to show Senku. "This one's water-based, but there are also silicone-based lubricants, too. 

"You just squeeze some on your hand, like so..." He put a generous dollop of the lubricant in the palm of his hand. "Then you rub it around your penis." He scooped up Senku's cute little cock, stroking up and down the length; Senku couldn't hold back a gasp at the feeling of his big brother's large, warm hand surrounding him. Xeno gave him a few strokes and Senku whined under his breath. "Doesn't it feel good?"

"Feels weird... wet and slimy..."

"It's like that at first. It'll get better as time goes on, though, I promise." He kept rubbing, occasionally teasing the head or hefting the soft, plump balls in his fingers. He felt Senku get progressively harder until his cock was twitching, hips unable to keep from rocking under the force of Xeno's strokes.

Senku whined and twitched under Xeno's ministrations. "Nn... Xeno, I'm gonna..."

"Feels like you're gonna pee, you said, right?" Senku nodded in a daze. "That's fine. Don't fight it. Just let it come out. Just come for me, Senku..."

With a small, strangled cry of _"Xeno—!"_ Senku did just that. Thin, watery ropes of newly-developed cum shot out of the tip of his cock, dribbling over Xeno's hand and down onto his sweatpants. The sight was nearly enough to make Xeno come, himself.

Senku fell backwards against Xeno's chest, suddenly feeling exhausted. "Wow..."

"Feels good, right?" Xeno smiled down at his little brother, clean hand running through his cowlicked strands. "It's healthy to do this regularly. Three or four times a week is fine. And like I said, you can do it yourself, or get someone else to give you a hand just like this."

"Do you do this, too?" Senku asked through panting breaths. "I wanna practice doing it..."

"Yes, I do... you can practice on me, if you want."

"Thanks!" Senku looked up at his big brother and, _fuck,_ the adoration on his face made Xeno want to ruin him.

"Yeah, yeah. Don't say I never did anything for you. Here, get off my lap, we'll make this easy for you." Senku stood on wobbly legs, giving Xeno the chance to push the waistband of his sweats down. He grabbed the pillow from the head of the bed and set it between his legs. "Go ahead and kneel on that, it'll be more comfortable for your knees."

With a nod, Senku settled between Xeno's legs. "Your penis is so much bigger than mine..."

"Well, I _am_ 12 years older than you, squirt." Senku pouted at the nickname and Xeno chuckled. "But as you get older, you'll get bigger too. Probably."

"Hey! Do you want me to practice or not?"

"Sure, sure..." Xeno picked up the bottle of lube. "Hold out your hands. You'll probably need both." Senku followed Xeno's command so obediently that it took everything in Xeno not to groan. He drizzled lube over one of Senku's palms and the boy rubbed them together to spread the fluid all over his hands. "Now, you saw what I did, right? You try now."

Senku's hands were surprisingly soft, given how often Xeno found him tinkering with his rocket. He bit down hard on the inside of his cheeks to hold back a groan at the feeling of the two small hands wrapping around his shaft. Two fingers eased his foreskin down, just like what Xeno had done to him, to tease under the skin and play with the slit. The other hand stroked up and down the length of his cock, from the base to just under the head.

"It smells funny," Senku mumbled; Xeno could feel precum oozing out of the slit and coating his fingertips. "Have you ever tasted it?"

"That's an _interesting_ question coming from an 11 year old," Xeno panted out. "If you're so curious, you could try it for yourself..."

Senku eyed his big brother with suspicion, but leaned forward and flicked his tongue across the tip. The way he recoiled at the taste made Xeno laugh out loud and ruffle his hair once more. "That's pretty much what I expected. It's not exactly pleasant, but I think it's worth the discomfort for the sake of seeing the other person come apart at the seams."

Senku leaned back on his haunches as he continued to move his hands. "Are you getting close?" he asked curiously. "I want to see what the difference is between your cum and mine."

"I'm getting there. You'll have to work a little harder than this to make me come." In all honesty, Xeno was close to blowing his load already, between the sight of his cute little brother jacking him off and the feeling of Senku's tongue against his cockhead. But he wasn't going to let Senku have the satisfaction of knowing that. 

Licking his lips, Senku moved one of his hands down to play with Xeno's balls. His other hand worked around Xeno's head, twisting and squeezing just on the edge of too much. The younger boy swallowed heavily before he seemed to come to a decision; and steeling his resolve, he leaned forward again and wrapped his mouth around the exposed head.

The sight and feeling of Senku's plush, sweet lips engulfing his cock was enough to push Xeno over the edge. The first blast coated Senku's tongue and he jerked back, coughing on the thick fluid; the second and third shots of cum sprayed across his face and into his hair. Xeno shuddered at the sight. "Sorry, Senku... didn't mean to," he mumbled half-heartedly. 

Xeno wrapped his own hand around his abandoned cock, stroking it firmly to push the last dregs of cum out of the length before it softened. "Stay here, I'll grab you a washcloth."

Senku nodded absently. It was amazing how different a grown-up's cum was—thick and gluey compared to Senku's thin, watery cum. He could still feel it, cloying in his mouth, no matter how many times he swallowed. He imagined he'd never quite get the smell out of his nose. Even when Xeno brought him a washcloth, it was like the thick fluid clung to his skin, invisible but always there.

“Now remember, Senku. Dad doesn’t find out about this, alright?” Xeno said firmly.

“Right. Can I… can I come practice with you again?”

Xeno had to bite the inside of his lip, but he nodded. “Sure, squirt. Just keep it between us.”

“I will, I promise!”

“Good.” Xeno smiled and reached out to ruffle Senku’s hair once more.

He’d known for ten years he was fucked… but he didn’t realize until that moment just how far _gone_ he was.

**Author's Note:**

> Feel free to join the Dr. Stone shipping server! All ships are welcome!  
> We have lots of events scheduled, RP our fave ships, share headcanons, etc!  
> [Click here to join us!](https://discord.gg/BfNeKvrTWp)


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